Analog Applications Journal
Personal Electronics
Optimal operating point of an LED
By Donald Schelle
Analog Field Applications
Relative Luminous Flux (a.u.)
Forward Current (mA)
Screen Size (inches)
Achieving optimal performance of an
Aspect Ratio
Conversion Constant
LED luminaire or LED backlight design
16:9
(inches2 to m2)
requires numerous trade-offs.
Understanding an LED’s power transfer
L 2  A × AY 
1
1
characteristics empowers intelligent
(1)
# LED min > S  X
× M V(disp) ×
×

choices regarding cost, power consumpK  A X2 + A Y 2 
1 − Vdisp ϕ V
tion, and weight. While most LED datasheets publish pertinent data that can
Max
Lumped
Lumen
Screen Area (m2)
be used to make these decisions, data
Brightness
Stackup Output of
may not be formatted in a way that is
(nits)
Losses Single LED
readily applicable to the chosen application. Optimal performance requires findUsing a conversion constant of K = 1550.0031 and the
ing pertinent information from manufacturer’s LED
design requirements listed above, the calculated minimum
datasheets and utilizing methods to capture, reformat and
number of LEDs is 35. While seven strings of five LEDs
analyze the data.
satisfies the design requirements, most LED driver ICs in
A relevant case study involves a typical tablet LCD
this market are tailored to drive only six strings of LEDs.
backlight application that drives a 10-inch display with a
Adjusting the LED count to 36 enables an off-the-shelf
16:9 aspect ratio. Driving the backlight, the LED chosen
LED driver. Assuming 100% driver efficiency, driving 36
for our example is the Nichia NNSW208CT[1]. Typical disLEDs at maximum brightness consumes 2.56 W of power.
plays in modern mobile devices emit approximately 650 nits
LED efficacy, color shift, and thermal properties are key
of light when driven at maximum brightness. Most of the
data metrics. Efficacy versus forward current is rarely proLED light produced is lost as it passes through the physivided in an LED datasheet. Tabulated efficacy data is also
cal elements integrated into the display (light diffuser,
difficult to find in specifications. Calculating this key metpolarizers, RGB color filter, touch-panel ITO, and so on).
ric is relatively easy using available IF versus VF and lumiModern display stack-ups loose approximately 95% of the
nosity versus IF curves. Also required is a typical lumen
light produced by the LED. This device in this case study
output at a given IF (8.4 lumens at IF = 20 mA). All
emits 10.398 lumens when driven at the recommended
required data is readily available in the manufacturers’
continuous drive current of 25 mA. Calculate the minidatasheets.
mum number of LEDs using Equation 1.
Start by importing/digitizing
the datasheet graphs (Figure 1)
Figure 1. Cornerstone plots used to derive the optimal LED operating point
into a spreadsheet using predefined increments of LED
Nichia (NNSW208CT)
100
current. Free software tools
4.5
speed the process and digitize
4.0
Y data on pre-determined X
3.5
increments[2], enabling the
calculations required to derive
3.0
efficacy.
10
2.5
2.0
1.5
1.0
8.4 Lumens at 20 mA
0.5
1
2.5
0.0
2.75
3
3.25
Forward Voltage (V)
Texas Instruments
3.5
0
20
40
60
80
Forward Current (mA)
22
100
AAJ 1Q 2015
Analog Applications Journal
Personal Electronics
Figure 2. LED flux output, power consumption and efficacy
can be calculated and plotted
40.0
Flux (lm)
30.0
20.0
10.0
0.0
0
20
40
60
80
100
0
20
40
60
80
100
0
20
40
60
80
100
500
Power (mW)
400
300
200
100
0
Efficacy (lm/W)
225
200
175
150
125
100
LED Forward Current (mA)
Once digitized and tabulated, LED flux output (ΦV),
LED power consumption (PLED) and efficacy (η) are calculated versus LED forward current (IF) (Figure 2). Peak
efficacy is reached at a relatively low forward current and
drops off steadily as forward current approaches the maximum rated amount.
Battery-powered applications greatly benefit by reducing these power requirements. Operating more LEDs at a
lower forward current results in a net reduction of power
for a given fixed-light output. Table 1 summarizes the original application requirements while comparing three alternative LED configurations.
Cost and mechanical volume requirements may limit the
final configuration; however, doubling the number of LEDs
yields a power savings of 160 mW. This equates to a 6.3%
net power reduction. Additionally, the backlight can be
operated at a much higher brightness (with increased
power consumption) when ambient light conditions (outdoors/daylight) dictate a brighter image.
Texas Instruments
Table 1. A comparative backlight design study
Number of
LEDs
Total Light
Output
(lm)
Total Power
Consumption of LED
Array
(W)
Net
Reduction
in Operating Power
(%)
32.6
373.2
2.65
–3.5
25
374.2
2.56
0
LED
­Operating
Point
(mA)
Decreasing Number of LEDs
24
Control
36
Increasing Number of LEDs
23
42 (16%)
21.2
373.2
2.50
2.2
54 (50%)
16.4
374.1
2.45
4.1
72 (100%)
12.2
374.1
2.40
6.3
AAJ 1Q 2015
Analog Applications Journal
Personal Electronics
Figure 3 highlights the increasing power savings trend
over a number of LED data points. Note that the LED
knob turns both ways. Overdriving each LED decreases
the total number required, yielding a less expensive display module; which is particularly beneficial when cost is
crucial.
Operating the LEDs at a reduced brightness requires
100% duty cycle and a reduced current. Driving the LEDs
using a traditional pulse-width modulation (PWM) architecture at maximum LED current yields no performance
improvements.
White-point shift at lower currents is a perceived complication for backlight applications. Modern LEDs exhibit
minimal to negligible color shift. Digitizing the LED’s colorshift parametrics (Figure 4) and superimposing a
MacAdam ellipse over the center of the operating range
highlights this point. A one-step MacAdam ellipse encompasses all LED colors when operating between forward
currents of 5 mA and 25 mA. Colors inside a one-step
MacAdam ellipse are perceived as the same to the average
observer.
Figure 3. LED power analysis shows lower
power with greater number of LEDs
Net Reduction in Operating Power (%)
10
5
0
–5
20
40
60
80
100
Total Number of LEDs
Figure 4. White point shift versus LED current
(Nichia NNSW208CT)
0.305
1 mA
5 mA
Chromaticity, Y
0.300
1 Step
2 Step
0.295
25 mA
35 mA
0.290
0.297
0.299
0.301
0.303
0.305
Chromaticity, X
Texas Instruments
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AAJ 1Q 2015
Analog Applications Journal
Personal Electronics
Figure 5. The LP8555 can power large matrices of LEDs
VBATT
2.7 to 20 V
L1
VOUT_A
7 to 28 V
D1
COUT_A
CIN_A
CVDD
SW_A
FB_A
VDD
LCD Display
VLDO
CVLDO
LP8555
LED Bank A
LEDA1
LEDA2
LEDA3
EN/VDDIO
EN
LEDA4
RPULL-UP
LEDA5
FSET/SDA
SCL
ISET/SCL
INT
PWM/INT
LEDA6
LEDB1
RPULL-UP
LED Bank B
SDA
LEDB2
LEDB3
VDDIO
LEDB4
LEDB5
LEDB6
FB_B
SW_B
GNDs
VBATT
2.7 to 20 V
L2
VOUT_B
7 to 28 V
D2
CIN_B
COUT_B
Conclusion
Powering a large array of LEDs is relatively easy using
an LED driver such as the LP8555 (Figure 5). This device
drives up to 96 LEDs, which is suitable for the largest of
mobile displays and capable of driving all the configurations mentioned above. Two-percent string-to-string
matching is a key metric to maintain a uniform image
quality. A dual-boost architecture maximizes electrical
­efficiency while minimizing the physical height of the
­associated inductors. Additionally, this device features 12
current-sink inputs, enabling shorter series-LED strings.
This allows the boost converters to power the LEDs at a
more efficient electrical operating point. Key features such
as adaptive dimming and content-adjustable backlight
­control (CABC) yield further electrical efficiency gains
over all operating modes.
For maximum power savings, the key objective is to tailor
the operating point of the LED to the most typical operating mode of the application. While LCD backlight applications have been the main focus, the concepts presented
here can be easily applied to any LED lighting application
that requires efficiency as a key performance metric.
References
1.Nichia NNSW208CT datasheet. Available :
www.nichia.co.jp/en/
2.Donald Schelle, Mark Brouwer, “Digitize graphical data
easily and accurately,” EDN, March 2013. Available:
www.edn.com/
Related Web sites
www.ti.com/1q15-LP8555
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25
AAJ 1Q 2015
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